Nuclear resonance scattering of Synchrotron Radiation
Nuclear resonance scattering (NRS) of synchrotron radiation describes a wide spectrum of different experimental techniques, such as:
Especially applications to samples under extreme conditions like high pressure, low/high temperature, high external magnetic field, confined geometries or tiny samples benefit most from the outstanding properties of synchrotron radiation.
The scientific topics being studied may roughly divided into hyperfine spectroscopy and (structural) dynamics.
- hyperfine spectroscopy
comprises the investigation of static and dynamic magnetic and electric properties such as electron densities, fields and field gradients of magnetic or electric origin.
- In transmission geometry poly- and single crystalline samples and amorphous specimens can be investigated by nuclear forward scattering (NFS).
- Surfaces, interfaces and multilayers will be investigated using nuclear reflectometry (NR).
- Single crystals allow one to determine
in addition the electric and magnetic structure by
nuclear Bragg diffraction (NBD).
- Magnetic and electric domains,
domains with roughness are accessible by
nuclear small angle scattering (NSAS)
- structural dynamics
comprises the investigation of element specific
Structural dynamics of "non-resonant" samples are accessible
- density of phonon states (DOS) and
- for DOS via inelastic scattering
with nuclear resonance energy analysis and
- for diffusion etc. via time interferometry.
- Basic and fundamental research
as longitudinal coherence, interferometry, nuclear and electronic interference, features of nuclear diffraction.
Theoretical principles of nuclear resonant scattering were worked out just after the discovery of the Mössbauer effect
(for a review see [Hyp.Int.123(2000)3]).
S.L. Ruby suggested in 1974 that synchrotron radiation could be used for a resonant excitation of nuclei. In the following years the theory of nuclear resonant scattering was developed by Yu.M. Kagan. Only ten years later - in 1984 - the first successful experimental proof of nuclear resonance scattering with synchrotron radiation was reported [Phys.Rev.Lett.54(1985)835].
The NRS project was one of the early scientific cases for the conception of the European synchrotron radiation source [ESRF Foundation Phase Report, Feb 1987].
Consequently, the ESRF machine has been designed with
- an electron energy of Ee = 6 GeV in order to reach
a photon energy of Eg = 14.412487(3) keV
(the transition energy of the 57Fe resonance)
in the fundamental of an undulator with 20 mm gap,
- a proper timing structure in order to observe
the 'delayed' g-rays of the nuclear decay
following the excitation of the nuclear levels
by the synchrotron radiation pulse.
Demands on the Beamline
The design goal of the nuclear resonance beamline
is described by the following points:
High brilliant hard x-ray beam with
for the excitation of the nuclear levels
of Mössbauer isotopes (see details)
||6 ... 30 keV
|High energy resolution of
|Pulsed time structure with
||D t =
These demands determined the design and the structure of the beamline. Main features are
- high energy resolution down to meV level
by 'electronic' monochromators and the
- ultra-high energy resolution down to neV level
by 'nuclear' monochromators, as well as the
- time resolution down to sub-ns level
- pulsed time structure of the synchrotron source.
A proper timing mode of the storage ring is a necessary requirement. Standard timing modes are e.g.
- 16-bunch mode (176 ns spacing
between adjacent bunches)
- 32-bunch mode (088 ns spacing)
- 01-bunch mode (2.8 ms spacing) and
- hybrid mode.
The following figure depicts the typical setup
for a nuclear resonant scattering (NRS) experiment.
The flash of SR light
- generated by the electrons in the storage ring
while passing the undulators
is monochromatized in two steps
and scattered by the sample.
The delayed signal seen by fast detectors is recorded by means of fast electronics. The radio frequency signal of the storage ring serves as timing reference (bunch clock). The photon flux is monitored constantly by an ionization chamber (IC).